Creating realistic concert lighting
If you want realistic results in cycles, there is no way around using spotlights and volume scatter with optimized render settings.

Here is a link to a similar question:
Volumetric Spot Light with Cycles

If you want to fake it, there is your way of doing it, however I would suggest to control the transparency of the "cone" with a gradient or ramp map.
Alternatively there is the camera node with the distance connector that can be used to control the transparency of the cone transparency (closer up the cone should be more transparent).

Unfortunately this will still give relatively amateurish results as the cone will always stay sharp at the edges, which spotlights dont do.

Another way would be to create an alpha texture with a faded gradient and use planes to fake the cone, unfortunately this will only work good if the result is not animated and doesnt change perspective.